Conventionally, in a hot rolling line for metal pieces (for example, steel, aluminum or copper), since the metal pieces have been extracted one by one from a heating furnace, there occurred various problems particularly in the finishing rolling process, as follows.
1) A biting failure at the front end of a metal piece.
2) A defective stamping at the rear end of a metal piece (a phenomenon that a corner at the rear end of a thin metal piece is bent).
3) A traveling problem of the front end of a metal piece occurring on a run-out table.
4) A defective dimension at the front and rear ends of metal pieces.
As means for solving the above-mentioned problems, there has been proposed a so-called endless rolling in which metal pieces to be rolled are joined at their rear and front ends before the finishing rolling, and continuously supplied to the fomoshinh rolling line to carry out the hot rolling.
As prior documents in this regard, a number of propositions can be found, for example, in Japanese Patent Laid-open Publication Nos. 60-244401, 61-144203, 62-234679, 4-89109, 4-89115 and 4-89110.
When carrying out the endless rolling of metal pieces, the following process has been generally performed. That is, firstly, on the entry side of a rolling equipment, a small gap is provided between a preceding metal piece and a succeeding metal piece at the ends thereof and these metal pieces are opposed substantially in parallel with each other. Further, a portion in the vicinity of the end of each metal piece is clamped and supported by clamps, and the end regions on the opposed faces of the metal pieces, which are portions to be joined, are heated by a heating means. The both metal pieces are then pressed against each other to be joined. In the joining method for metal pieces using such a process, various disadvantages which will be described below have still remained and an improvement in this regard has been desired.
1) In the joining form of this type, there is provided an inductor having a pair of magnetic poles vertically sandwiching the portions to be joined in the metal pieces. With the inductor, an alternating magnetic field running through the metal pieces in the thickness direction thereof is applied, and a surface layer at the portions to be joined, or on the opposed surfaces in particular is intensively heated by the induced current generated at this time. The induced current is, however, hard to flow at corners of the front and rear ends of the metal pieces, and the heating temperature at the portions to be joined thus gradually lowers toward the wide ends. Thus, there is such a disadvantage that the portions to be joined cannot be joined over the full width thereof when pressing the metal pieces against each other.
In this case, the wide ends having a low temperature function as a resistance when pressing the metal pieces against each other, if the temperature at the portions to be joined does not reach a target heating temperature, so that a pressing apparatus having a capacity above a necessary level must be installed. Also, since a sufficient joining strength cannot be secured, the joined portions are gradually separated as the rolling proceeds and the metal plate are ruptured so that a serious accident may occur. In order to solve such a problem, it is most effective to continue the heating until the temperature at the corners of the steel pieces reaches a target value. However, since the temperature in regions other than the corners (i.e., central regions in the plate width direction) reaches a melting temperature and the metal pieces are melted down in these regions, excellent joined portions cannot be obtained. In addition, such a heating may deteriorate the surface quality of the plate at the portions after rolling which correspond to the joining portions or other portions close thereto, and the input electrical power has to be increased as well.
2) In the induction heating method using the inductor, since a variation rate of magnetic fluxes (a variation rate of the number of magnetic fluxes) is proportional to the current to be induced, the variation rate becomes large as the number of magnetic fluxes running through the metal pieces is large at the peak of the alternating current which flows in a coil of the inductor, thus increasing the scale of the current. Further, since a vertical component in the magnetic flux generated by the inductor for the surface of the magnetic pieces advantageously contributes to the generation of the induced current, the induced current can be increased as the magnetic flux running through the magnetic pieces becomes vertical.
However, in the metal piece heating and joining to which the induction heating method is applied, if the positional relationship between the inductor and the ends of the respective metal pieces (the position of the inductor in the longitudinal direction of the metal pieces in particular) is inadequate, the number of magnetic fluxes running through the metal pieces is insufficient and the vertical component of the magnetic flux is also disadvantageously reduced. Further, for example, in a conventional method disclosed in Japanese Patent Laid-open Publication No. 62-234679 mentioned above, this kind of disadvantage is not considered therein, and the satisfactory heating speed cannot be obtained depending on the positional relationship between the inductor and each metal piece, involving a prolongation of the heating time. Also, since the preceding metal piece and the succeeding metal piece are not uniformly and equally heated, an excellent joining cannot be realized.
3) When heating the metal pieces, if the preceding metal piece and the succeeding metal piece are different in the initial temperature or in the plate thickness, or if the metal pieces having different melting points are heated, an appropriate heating cannot be performed in accordance with each metal piece, and a sufficient strength cannot be given to the joined metal pieces. In such a case, the plates may be ruptured from their joined portions during rolling, resulting in a serious accident.
4) There is proposed a joining method employing a so-called non-contact heating in which the preceding and succeeding metal pieces are heated with a space therebetween and the up-setting is then performed for joining (Japanese Patent Laid-open Publication No. 60-244401). According to this method, although the uniform magnetic flux must be given to the metal pieces over the full width thereof, the uniform magnetic flux is restrictedly given to the width of not more than 1000 mm mainly from a viewpoint of design in the power supply of the inductor. In the case of a larger width, for example, a width of 1900 mm, a pair of inductors are required (the limit in the capacity of one inductor is 2000 through 3000 kW, and practical use of an inductor which has a capacity of not less than 4000 kW and can deal with the width of 1900 mm is difficult). It is therefore difficult to give a uniform magnetic flux running through the metal piece over the full width thereof.
5) For heating the preceding and succeeding metal pieces in a relatively short time for joining, there is an induction heating rolling method, as a heating means, in which air-core type coil is used to induction-heat the rear end of the preceding metal piece and the front end of the succeeding metal piece and the both ends of these metal pieces are then pressed against each other for joining (Japanese Patent Laid-open Publication No. 60-244401). In this method, however, since the metal pieces to be joined are inserted into the coil and heated, this method cannot be applied to a metal piece whose dimension is larger than the inner dimension of the coil. On the other hand, in the case where both metal pieces have a small width, such a problem does not occur though a part of the magnetic fluxes does not contribute to heat the metal pieces, thereby deteriorating the heating efficiency for the input power. Further, in connection with the above item 4), in the case where the metal pieces are heated using a plurality of inductors, since the magnetic flux is not generated from a portion corresponding to a part between inductors adjacent to each other, the magnetic flux running through the metal piece is locally decreased and the temperature rise at this portion is insufficient. It is therefore important to make a space between the adjacent inductors as small as possible, but the space between the adjacent inductors cannot be reduced at a stage of securing facilities. Thus, the temperature distribution of the metal pieces in the width direction thereof inevitably becomes uneven.
6) Although a preferred temperature at the opposed faces (the end faces) of the metal pieces can be usually within a range of approximately 1350 to 1400.degree. C., in the case where the metal pieces joined at such a temperature are rolled by a finishing rolling mill composed of a plurality of stands which functions after joining with a draft percentage increased by 10 times or more, all kinds of steel cannot be rolled without causing rupture at the joining portions until the completion of rolling.
In the endless rolling of the metal pieces, since the timing of the processing for joining the metal pieces must match with that of the rolling process, it is general to provide a movable joining apparatus placed on the entry side of a group of finishing rolling mills so that it can follow the movement of the steel pieces, or to provide an apparatus such as a looper having a timing buffer function between the joining apparatus and the rolling facilities. This involves an extension of the line or provision of new apparatuses, thereby disadvantageously increasing the facility cost. However, in regard of this problem, a proposition disclosed, e.g., in Japanese Patent Laid-open Publication No. 4-89120 has been given and the problem has been already improved.
The present invention intends to achieve the following objects.
1) On the entry side of the hot finishing rolling facilities, the preceding metal piece and the succeeding metal piece are uniformly heated over the full width thereof and, when pressing the metal pieces against each other, the joining is carried out until a sufficient strength can be obtained (until such a strength that the plates are not ruptured during rolling can be secured), thereby performing a stable rolling operation.
2) The both metal pieces are heated in an appropriate range of temperature and joined with a sufficient strength being given thereto irrespective of temperature or thickness of the metal pieces to be joined or kinds of the plates.
3) Even if a plurality of inductors for generating magnetic fluxes are used, a stable heating and joining are realized by making uniform phases of currents to be supplied to the respective inductors.
4) The metal pieces are heated and joined with a good efficiency in a short time irrespective of difference in sizes of the metal pieces and widths of the same in particular.
5) Irrespective of the kinds of the metal pieces to be joined, an excellent joining is realized by giving such an advantageous heating condition that problems such as rupture of the plates do not occur in the succeeding finishing rolling process.